Mexico City is sinking at an alarming rate, with some areas subsiding by more than 2cm a month, according to data from the Nisar satellite, one of the most powerful radar systems ever launched into space. The joint Nasa-Indian Space Research Organisation mission is providing unprecedented real-time tracking of the phenomenon, which has been ongoing for over a century.
The sinking, first documented in 1925, is caused by centuries of groundwater extraction. The city was built on an ancient lake bed, and as water is pumped from the aquifer below, the clay-like soil compacts under the weight of the metropolis. The aquifer still supplies about half of the capital's water, and the water table is contracting by roughly 40cm a year.
Nisar's radar can detect minute changes in Earth's surface, even through thick vegetation or cloud cover. Scientists say it offers a more detailed view of subsidence across different land types than any previous space-based sensor, including previously hard-to-study areas on the city's outskirts.
The impact is visible across the city of about 22 million people: tilting historic buildings, warped roads, and damage to the underground metro system. The Angel of Independence monument on Paseo de la Reforma has had 14 steps added to its base since its completion in 1910 as the ground around it has sunk.
Experts warn that the sinking creates a vicious cycle, as ageing water pipes crack and break, leading to an estimated 40% water loss from leakage. Combined with low rainfall linked to the climate crisis, the situation could worsen. Nisar's capabilities extend beyond urban subsidence to monitoring volcanoes, earthquakes, landslides, and climate change.
